'''  Purpose:  introduce variable usage, assignment and hand-checking of code.
'''

n = 3               # memory is reserved for variable n. the memory is set to 3

print( "n =", n )   # a string and the value of a variable are printed

s = 2 * n           # memory is reserved for variable s. the memory is set
                    # to product of 2 and the value currently stored for in
                    # variable n (3). 2 * 3 equals 6, so s is set to 6

print( "s =", s )   # a string and the value of a variable are printed

t = s + 4           # memory is reserved for variable s. the memory is set
                    # to sum of the value currently stored for variable
                    # s (6) and 4. 6 + 4 equals 10, so t is set to 10

print( "t =", t )   # a string and the value of a variable are printed

u = t // 2          # memory is reserved for variable u. the memory is set
                    # to integer quotient of the value currently stored for 
                    # variable t (10) divided by 2. 10 // 2 equals 5, so u
                    # is set to 5

print( "u =", u )   # a string and the value of a variable are printed

v = u - n           # memory is reserved for variable v. the memory is set
                    # to produce the difference of the values currently stored
                    # for variable u (5) and n (3) 5 - 3 equals 2, so v
                    # is set to 2

print( "v =", v )   # a string and the value of a variable are printed

u = -5              # there is already memory set aside for u. so that
                    # the memory is now set to 5.  the assignment only updates
                    # u.  so none of the memory for the other variables 
                    # is changed.

print( "u =", u )   # a string and the value of a variable are printed

print( "v =", v )   # because v is unaffected by the assignment to u, v's
                    # value of 2 is printed.